The objective of the proposed study is to evaluate the effects of HLA compatibility, transferrin alleles, and other genes on reproduction. Our population-based study will be conducted in the Hutterites, a religious isolate that lives communally and proscribes contraception. During field trips to Hutterite colonies in South Dakota, blood samples for genetic analyses will be collected from all cooperative adults. All samples will be typed for HLA-A,-B,-C,-DR,-DQ,ABO,Rh,Gm,Km, transferrin. In selected couples studies of HLA-DP sharing and presence of maternal plasma blocking factors and anti-lymphocyte antibodies will be performed. The specific hypothesis tested is whether normal pregnancy requires maternal recognition of paternally-derived histocompatibility antigens carried by the fetus (i.e., histoincompatibility). We seek to: 1) identify gene(s) or region within the MHC associated with reduced fertility in couples sharing HLA antigens; 2) investigate the relationship between reproductive histories and presence of anti-lymphocyte antibodies and/or maternal plasma blocking factors, in couples sharing and not sharing HLA antigens; 3) examine interactive effects between HLA compatibility and other genes known to affect immune responsiveness or fertility (namely ABO, Rh, Gm, Km, Transferrin) and, 4) evaluate the effects of HLA compatibility on genotype or haplotype segregation ratios and sex ratios among offspring. Elucidating the immunological mechanisms involved in normal pregnancy is necessary for understanding immunopathological processes causing spontaneous abortion in man. Thus, our studies may help delineate a subset of women who experience repetitive spontaneous abortions who may potentially benefit from immunotherapy.